Introduction
According to early cosmology -and to the Jewish, Christian and Muslim faiths- the universe started at a finite time, not a very long time ago in the past. According to the Book of Genesis and St Augustine, the creation occurred around 5,000 BC. This, of course, is in complete contradiction with modern science.
Aristotle and many other Greek philosophers did not like the concept of creation because it implied the action of a divinity. They believed that the universe and man had existed forever. As long as man believed that the universe was unchanging and static, it did not import if it had a beginning or not.
In 1929 Edwin Hubble found that all distant galaxies are moving away from us, that is, the universe is expanding. From this it is obvious that at one time the galaxies were closer to each other or, even better, that all the galaxies were concentrated in a single point many thousand million years ago (ten to twenty). At that moment the density of this single point universe must have been infinite.
The beginning of the universe occurred with an explosion described now as "The Big Bang." There and then all the laws of physics cease to be applicable and, what happened before, and what happened afterwards, could no be predicted.
In a stable universe the beginning of the universe requires God's intervention, and this could have taken place anytime in the past. If we agree, on the other hand, that the universe is expanding a beginning at a well-defined time is required. In this case we can still believe that God created the universe at the time of the big bang, but He could not have chosen the moment.
The universe as seen by the Greeks
As late as the 15th century, most people thought that the earth was flat
even if, around 430 BC, Aristotle wrote in his book "On the Heavens"
that, in his opinion, the earth was a sphere. This was due to two observations:
- During the eclipses of the moon (when the earth comes between the sun
and the moon) the earth shadow on the moon is always a circle.
- When a boat appears on the horizon, one first sees the topmast then the
hull.
The Greeks had also noticed that although most stars appeared to move together in the sky, five celestial objects -in addition to the moon- did not. They moved mostly from the east to the west but, sometime, they would move in the other direction. The Greeks called them "planets" ("wanderers" in Greek). They knew only five planets -Mercury, Venus, Mars, Jupiter and Saturn- because the others could not be seen with the naked eyes.
Aristotle thought that the earth was stationary with the sun, the moon, the planets, and the stars moving in circular orbits around it. His belief was not based on observation but on mystical reason. He thought that the earth was at the centre of the universe, and a circle is a perfect figure.
Later on Ptolemy proposed a model of the universe formed of eight spheres, each with a larger radius that the one before, rotating around the earth, the centre of the universe. The eighth sphere, on which stars occupied fixed positions, was the boundary of the universe. The planets were on the inner spheres but they had no fixed positions like the stars. On the opposite they moved in small circle called "epicycle" to account for their strange pathway. This theory was accepted more or less by everybody during many centuries although it did not match all observations. The Catholic Church accepted this theory because there were room for Heaven outside the eighth sphere.
The universe from the 16th to the 19th century
In 1514 the Polish priest Nicolaus Copernicus suggested that the sun -and not the earth- was stationary at the centre of the solar system, and that the earth and the planets rotated around it on circular orbits. It was simpler that Ptolemy's theory but it still did not match all observations.
In 1609 Galileo Galilei, using the recently invented telescope, found that Jupiter had some moons orbiting around it. This cancelled all previous theories that implied that every celestial body rotated around the earth.
More or less at the same time Kepler suggested that the planets were moving on elliptical orbits and not on circular ones. Now the combined Galileo and Kepler theories matched the observations
It was Sir Isaac Newton in 1687 that proposed the theoretical justification
why the planets move on elliptical orbits around the sun. He claimed that
the force that makes the planets move on elliptical orbits is gravity, the
same force that makes objects fall on the earth as they do. He also provided
the mathematical laws that allow us to forecast the effect of gravity. In
particular, these laws allowed him to calculate the elliptical orbits of
celestial bodies as foreseen by Kepler. As a result, there was no more reason
to believe that the universe has boundaries, and that the sun, and our solar
system, is the centre of the universe.